The preparation of multilayered ceramic hybrid circuits is well-known. Such composites are in extensive use in the semiconductor industry for the fabrication of, for example, substrate carriers for the mounting of semiconductor or integrated circuit chips, laminated ceramic capacitors, or the like. These products have been traditionally produced by either a thick film printing method, or by what is known as a green sheet lamination method.
The thick film printing method comprises the formulation of a multilayered structure based upon a fired ceramic base. The multilayered structure is achieved by the sequential screen printing of conductor and insulator inks comprised, respectively, of metal or metal oxide powders and ceramic powders, usually formulated in an organic polymeric binder. These coatings are selectively applied in consecutive fashion. This method suffers from the drawbacks that repetitive applications of the insulator or dielectric material must be applied to achieve each layer and the resulting substrate must be fired after each printing process. The extensive cycling of such a method adds undesired time and expense to fabrication and the ultimate cost of the product. It is moreover noted that the thick film printing method is limited in the density that can be achieved with interconnect circuitry, and is likewise prone to low production yields. Other drawbacks include ceramic layer hermeticity that results directly from the use of screen printing methods to form the various layers, and the inability of the bonding agents conventionally employed in metal pastes to function acceptably in the nitrogen firing applications that are required to avoid undesired oxidation of the conductor metal.
The green sheet lamination method comprises the printing of desired metal circuits on individual green ceramic sheets, which sheets are then stacked and successively laminated and thereafter co-fired to form a monolithic interconnect structure or package. This method generally may commence with the preparation of the ceramic green tape by, for example, a doctor blade casting process from a slurry containing a mixture of ceramic powders, thermoplastic resin, solvents, and other additives, such as dispersants and plasticizers. Vinyl polymers such as polyvinylbutyral (PVB) are commonly used in the resin system for the formation of the tape. After formation, the green tape is then blanked into sheets and registration holes are punched. Via holes which serve as vertical interconnects between layers in the final package may be punched using fixed tooling or corresponding adjustable equipment. The holes are then filled and circuit trace patterns are printed using the desired metallization compositions. After the individual sheets are prepared, they are stacked in the proper sequence and laminated to form the composite laminate. The laminate is then fired to decompose and remove the organic binder and to sinter the ceramic and metal particles, to thereby form the dense body containing the desired three-dimensional wiring pattern.
The preparation of multilayered ceramic circuit structures involving the green sheet method is extensively treated in the patent literature. Thus, U.S. Pat. No. 3,770,529 to Anderson discloses the preparation of such structures and specifically relates to the machining of the green sheets by radiation beams. U.S. Pat. No. 3,852,877 to Ahn et al. is specifically directed to a metallizing coating of specific composition. U.S. Pat. No. 4,861,646 to Barringer et al. discloses a particular glass ceramic composition binder system and metal "ink" for the preparation of improved green sheets. U.S. Pat. No. 4,871,608 to Kondo et al. relates to a particular conductor paste which includes copper/copper oxide and one of MnO.sub.2 and Ag.sub.2 O to limit thermal contraction of the conductor paste during firing.
Of the stages involved in the fabrication of multilayered ceramic substrates, attention has focused on the heating of the laminated structure to form the final sintered article. As can be appreciated, the heating procedure seeks to achieve distinct objectives, among them complete burn-out of the organic polymeric binder without the retention of carbonaceous residues or undesired porosity in the final article, and unwanted oxidation or reduction of the ceramic and metal components of the composite. The difficulty in the coordination of the heating program has been the balancing of the need for sufficient oxidation to remove the binder completely with the need for either a neutral or reducing atmosphere, as the case may be, to maintain or convert to the metallic state, the conductive material while retaining the oxidic state of the surrounding ceramic materials during high temperature baking and sintering.
The following patents are noted for their relevance to the heat treatment of multilayered ceramic circuits prepared with ceramic green sheets. Thus, U.S. Pat. No. 4,153,491 to Swiss et al. accelerates sintering of a multilayered ceramic hybrid circuit by eliminating a separate binder burn-off step. The patentees appear to rely on a ceramic green sheet based on high alumina content particles having average particle sizes greater than one micron, and close particle size distribution.
U.S. Pat. No. 4,877,555 to Yuhaku et al. discloses a conductor paste that comprises CuO as a main inorganic component, with an additive selected from Cu.sub.2 O and CuO. Yuhaku et al. teach the preparation of their multilayer structure by the disposition of their conductor paste and a dielectric paste in a predetermined sequence, followed by the heat treatment of the resulting structure, first to remove the binder, then to metallize the internal conductor and lastly, to fire the final product. The first heat treatment is performed in air, the metallization is performed in a reducing atmosphere, and the firing step is performed in a neutral atmosphere. The Yuhaku et al. disclosure seeks to control shrinkage and expansion of the copper conductor by varying the amounts of the above-noted inorganic components. The Yuhaku et al. invention is purported to permit the use of a relatively thin conductor layer and dielectric layer in the preparation of the composite therein. This result is desired in the instance of small-sized large capacitance capacitors.
In U.S. Pat. No. 4,863,683 to Nakatani et al., a method for preparing a multilayer conductor is disclosed where the conductor metals are provided as oxides which are converted to the metallic state during heating. Nakatani et al. first heat the composite to burn out the binder, and then heat the body in a reducing atmosphere to reduce the metal oxides. Both the binder burn-out and reduction heating steps are performed at temperatures below the sintering temperature of the glass or ceramic components of the insulating paste, to encourage conversion of the metal oxides of the conductor layers to the metallic state. Thereafter, the body is heated in a neutral atmosphere such as nitrogen to sinter the body to form the final multilayered article. Nakatani et al. are similar to Yuhaku et al. in their general use of a three-stage heating schedule, although they differ as to the compositions of their conductor pastes and the details of their heating steps.
U.S. Pat. No. 4,234,367 to Herron et al. employs an atmosphere of H.sub.2 and H.sub.2 O at temperatures below the melting point of copper, i.e., of up to about 785.degree..+-.10.degree. C. Sintering, however, is conducted in an inert atmosphere such as N.sub.2, and H.sub.2 O is not present. U.S. Pat. No. 4,504,339 to Kamehara et al. is similar to Herron et al. in that water vapor is included in the binder burn-off atmosphere. In Kamehara et al., water vapor is expressly excluded from the firing or sintering atmosphere, to avoid oxidation of the conductive copper pattern. Finally, U.S. Pat. No. 4,891,246 to McEwen et al. relates to the use of a firing atmosphere that includes CO/CO.sub.2 and water in combination, and distinguishes the use of H.sub.2 O alone. McEwen et al. is particularly noteworthy for its review of the prior art, which review is incorporated herein by reference.
From the above review and as particularly set forth in Yuhaku et al. and Nakatani et al., the heating program for these multilayered ceramic structures has generally been conducted in three stages, as follows: a first binder burn-out stage; a second reducing stage; and a third firing or sintering stage.
An additional concern that has been noted and is not addressed in the prior art, however, has to do with the staining of the ceramic phase of the structure by copper inclusions. More particularly, ionic copper is believed to diffuse into the ceramic/glass phase of the composite during binder burn-out and the early part of the sintering step, and during sintering, is believed to be reduced to a lower oxidation state which exhibits the characteristic purple or pink color of the metal. This reduction is believed to occur as a result of the low oxidation potential of the nitrogen atmosphere present during the sintering step.
As the bulk of the copper needs to be protected from oxidation at this stage, the addition of oxygen is generally avoided. Thus, the modulation of the composition of the firing atmosphere to achieve the desired combination of properties is further complicated by the unacceptable appearance that results from the unwanted diffusion of copper. A need therefore exists to reduce or eliminate staining of the composite article while maintaining or improving product integrity, conductivity and other desirable performance properties thereof.